Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $238.80   3.123%   $246.2577   $246.26   $246.25   $246.30  
  Purchase #2 $8.58   4.387%   $8.9564   $8.96   $8.95   $9.00  
  Purchase #3 $68.94   3.372%   $71.2647   $71.26   $71.25   $71.30  
  Purchase #4 $124.78   13.607%   $141.7588   $141.76   $141.75   $141.80  
  Purchase #5 $84.62   4.840%   $88.7156   $88.72   $88.70   $88.70  
  Purchase #6 $100.96   12.614%   $113.6951   $113.70   $113.70   $113.70  
  Purchase #7 $137.78   13.399%   $156.2411   $156.24   $156.25   $156.20  
  7 purchase totals = $764.46       $826.8894   $826.90   $826.85   $827.00